1. Field of the Invention
The present invention relates to a Rosen type piezoelectric transformer constituting a stabilizer for cold cathode fluorescent lamps used for a back light of a liquid crystal display (LCD) device. More particularly, the present invention relates to a Rosen type piezoelectric transformer with multiple output electrodes for lighting a plurality of cold cathode fluorescent lamps at the same time by forming multiple output electrodes as one body, and a stabilizer for multiple light sources using such Rosen type piezoelectric transformer.
2. Description of the Prior Art
As is well known to those skilled in the art, a fluorescent lamp needs high voltage at initial lighting in order to emit the required thermal electrons for an electric discharge. After electric discharge, the voltage impressed on the fluorescent lamp has negative resistance characteristics reduced when the lamp current increases.
Especially, a cold cathode fluorescent lamp that is used for a backlight of a liquid crystal display (LCD) device is a light source using cold emission (electron emission caused by forcing a strong electric field to a cathode surface). In order to light the backlight comprised of the cold cathode fluorescent lamp, about 500 V is needed. Generally, a step-up transformer is used to convert a commercial AC power source into the driving voltage.
The stabilizer is a lamp lighting device that applies the driving voltage for lighting the fluorescent lamp with the above mentioned properties, and comprises, as shown in FIG. 7, a rectifying section 621 which rectifies an AC commercial source 61 to DC voltage in some level, a power factor compensation section 622 for compensating the power factor caused by the fluctuation of the AC commercial voltage source in respect of the output voltage of the rectifying section 621, an AC voltage generating section 623 for converting the DC voltage inputted through the power factor compensation section 622 into a proposed AC voltage of high frequency, and a resonating section 624 for resonating with the output voltage of the AC voltage generating section 623 and supplying the AC voltage to the fluorescent lamp 63.
As above-mentioned, the AC voltage generating section 623 is called an inverter.
The resonating section 624 is conventionally constructed to generate the resonance by the inductance of a winding type transformer and the capacitance of a resonance capacitor in connection with the winding type transformer. But the winding type transformer has some problems in that it produces a high rate of poor quality and the credibility of the stabilizer is reduced because the deviation of the quality of core is excessive. For this reason, the winding type transformer has recently been replaced with a piezoelectric transformer such that the resonance is generated by the inductance of the inductor with the capacitance of the piezoelectric transformer.
But the above statement is the case of connecting one fluorescent lamp to one stabilizer. On the other hand, in a case of lighting more than two fluorescent amps by connecting the fluorescent lamps into one stabilizer, if one of numerous fluorescent lamps is lighted, other fluorescent lamps connected to the wound-type transformer in parallel are not lighted because a lower voltage is supplied to the other fluorescent lamps by lighting the one fluorescent lamp.
Accordingly, in the case of connecting two or more fluorescent lamps to one stabilizer, as shown in FIGS. 8 and 9, the ballast capacitor is directly connected between fluorescent lamps 72, 82 and transformers 712, 812 such that the voltage required for initial lighting is impressed on the fluorescent lamps 72, 82 even if a certain fluorescent lamp is lighted ahead of the fluorescent lamps 72, 82.
Hence, the conventional stabilizer used for a plurality of light sources is constructed as shown in FIGS. 8 and 9.
As is shown in FIG. 8, the stabilizer 71, for a plurality of fluorescent lamps having a wound-type transformer, comprises a circuit section 711, which includes the rectifying section 621, the power compensating section 622, the AC voltage generating section 623, and the resonance capacitor of the resonating section 624, the wound-type transformer 712, the input electrode of which is connected to the resonance capacitor of the circuit section 711, and a plurality of ballast capacitors 713 that are connected to an output electrode of the wound-type transformer 712 in parallel. The number of ballast capacitors is the same as the number of lamps. Furthermore, a plurality of fluorescent lamps 72 is connected to the other terminal of the plurality of ballast capacitors 713 of the stabilizer 71, respectively.
As is shown in FIG. 9, the stabilizer 81 for a plurality of fluorescent lamps having a piezoelectric transformer comprises a circuit section 811, which includes the rectifying section 621, the power compensating section 622, the AC voltage generating section 623, and the resonance capacitor of the resonating section 624, the piezoelectric transformer 812, the input electrode of which is connected to a resonance capacitor of the circuit section 811, and a plurality of ballast capacitors 813 that are connected to an output electrode of the piezoelectric transformer 812 in parallel. Furthermore, the plurality of fluorescent lamps 82 is connected to the other terminal of the plurality of ballast capacitors 813 of the stabilizer 81, respectively.
In the stabilizer above, because AC voltage supplied by the wound-type transformer 712 or the piezoelectric transformer 812 is divided equally among the plurality of fluorescent lamps via the ballast capacitor 713,813, the plurality of fluorescent lamps can be lighted at the same time. The ballast capacitor 713,813 has a high withstand voltage characteristic, and its size is also big.
Accordingly, as is shown in FIG. 8 or FIG. 9, because the stabilizer needs the ballast capacitors in proportion to the number of the connected fluorescent lamps, the volume and the number of components increase, and the cost increases also.
Thus, in the case of constituting a stabilizer as mentioned above, there have been some problems in that the number of components and the volume increases and the production costs rise because the number of ballast capacitors required is proportionate to the number of connected fluorescent lamps.
Accordingly, an object of the present invention is to provide a Rosen type piezoelectric transformer having a plurality of output electrodes with uniform output properties which make the fluorescent lamp light stably, the operation be simple, and the cost be reduced.
The present invention provides a Rosen type piezoelectric transformer with multiple output electrodes, being used for a stabilizer to light a plurality of cold cathode fluorescent lamps, comprising: a piezoelectric plate; an input electrode formed at an upper surface of the plate; a plurality of output electrodes having an identical band shape, arranged in equal distance in the longitudinal direction of the plate at an outer side of the input electrode, and connected electrically to one ends of the plurality of the cold cathode fluorescent lamps, respectively; and a common electrode formed at a bottom surface of the plate, confronting to the input electrode, commonly connected to ground of the stabilizer and the other ends of the plurality of the cold cathode fluorescent lamps.